This invention is concerned with an apparatus for producing compact polycrystalline ingots of InP, GaP, GaAs and the like having a defined geometric form.
Single-crystals of InP and GaP are pulled from melted-down polycrystalline material of stoichiometric composition in vertical pulling apparatus. The material to be used for this purpose must first be brought into suitable form or comminuted.
Single-crystals of gallium phosphide, such as are required for the manufacture of light-emitting diodes, can be pulled from a gallium phosphide melt by the known protected melt process. The polycrystalline gallium phosphide required for the melt can be prepared directly from the elements. According to the method described therein, a gallium phosphide bar crystallizes in a trough-shaped boat if the melt is allowed to solidify with unidirectional orientation.
In contrast to the melting crucibles in single-crystal pulling equipment, the bars obtained in the synthesis of the polycrystalline material do not have circular cross section. To charge the melting crucible, the bars must therefore be broken into pieces, which are then melted down in a crucible of circular cross section under a B.sub.2 O.sub.3 pellet. In this arrangement, however, the running of the B.sub.2 O.sub.3 melt between the GaP pieces cannot be avoided. As a result, a muddy, opaque B.sub.2 O.sub.3 protection melt is obtained after the first melting down. One must therefore prepare, prior to the crystal-pulling proper, in a separate operation called "regulus melting", a compact ingot of the semiconductor material, which has exactly the same shape as the melting crucible. The clear B.sub.2 O.sub.3 melt, which is necessary for growing single crystals is obtained as a cover layer on the gallium phosphide melt, only after this second melting operation under a B.sub.2 O.sub.3 pellet, in which the compact GaP ingot is employed.